Sources of information

Wave speed and gravity are both linked with density of materials. Exam tip: treat both as indirect clues.

Meteorites give clues about early solar system materials. Exam tip: treat them as indirect compositional evidence.

Mining gives direct information about only the shallow parts of Earth. Mines are made to obtain useful materials such as coal, metals, and ores, but people cannot safely dig through thousands of kilometres of rock. Therefore, mining is an important direct source of information, yet its reach is very limited compared with the size of Earth. The correct choice is the upper crust, where most mining activity takes place.

The inner core, outer core, and lower mantle lie far below the depth reached by mines. Information about these regions must come mainly from indirect evidence, especially seismic waves, gravity observations, and the study of Earth’s magnetic field. Thus option C follows because mining can reveal rocks and conditions near the surface, not the deep interior.

Lava comes from inside but it is not a direct sample of the core. Exam tip: understand the limit of direct sources.

Artificial explosions create waves used to study rocks below. Exam tip: consider it a controlled indirect method.

Travel time of waves helps infer distance and layers. Exam tip: connect travel time with thickness.

A geomagnetic survey studies variations in Earth’s magnetic field. Such variations provide clues about magnetic and iron-rich materials inside Earth, especially materials associated with the core. Seismic waves are more useful for finding the physical state and density of internal layers, not their magnetic properties. Exam tip: for questions on magnetic properties, link the evidence to geomagnetism.

Temperature can be measured with depth in boreholes. Exam tip: remember it as direct but limited source.

The direct answer is C, seismogram. A seismograph is the instrument that detects and records ground vibrations caused by earthquakes. The written or drawn record produced by that instrument is called a seismogram. This instrument-record distinction is the main idea. Option A, map, is a representation of places or features on Earth and does not specifically record seismic waves. Option B, bar graph, compares quantities using bars; it is a general graph, not the special earthquake record. Option C is correct because a seismogram shows the movement of the ground or the arrival and strength of seismic waves over time. Option D, hydrograph, normally shows changes in water flow or river discharge, so it is unrelated to earthquake recording. Therefore, do not confuse the device with its output: seismograph is the machine, while seismogram is the record. Memory cue: the instrument ends in “-graph” and its earthquake record is a “seismogram.”

S waves do not travel in liquids so their absence indicates liquid. Exam tip: this feature of S waves is very important.

In rigid and dense media wave speed can change and often increases. Exam tip: link speed with nature of medium.

Temperature and pressure are very high at depth. Exam tip: this explains the limit of direct study.

Rock samples provide actual material so this is a direct method. Exam tip: write sample based method as direct.

Earthquake waves indicate state and structure of materials. Exam tip: treat it as physical evidence.

Magnetic field gives a clue about metallic materials. Exam tip: remember this for metallic nature of core.

Wave speed direction and shadow zones indicate layers. Exam tip: consider seismic behavior as main evidence.

Gravity study indicates internal density differences. Exam tip: eliminate unrelated options in matching questions.

Medium capacity of waves helps infer solid and liquid states. Exam tip: study medium and state together.

The direct answer is Option B: oceanic crust. Rocks collected from the ocean floor are part of, or represent, the material forming the oceanic crust. By studying their minerals, age, chemical composition and magnetic patterns, geographers and geologists learn about the crust beneath oceans and processes such as seafloor spreading. Option B is correct because the question asks what information these rocks provide, and their most relevant information concerns oceanic crust. Option A, only wind speed, is a weather measurement and cannot be determined from the solid rocks of the ocean floor. Option C, only population, is a human-geography topic and is not recorded in rock material. Option D, only clouds, concerns the atmosphere and cannot be inferred as the main information from ocean-floor rocks. The word ‘only’ also makes these choices especially clearly wrong. Exam cue: rocks from a particular part of Earth usually tell us about that part’s material, age and history; ocean-floor rocks therefore point to oceanic crust.

The direct answer is A, change in the speed of seismic waves. Earth’s crust and mantle are different layers with different materials, densities, temperatures, and physical properties. When an earthquake wave reaches a boundary between materials, its speed and direction can change. The boundary between the crust and mantle is called the Mohorovičić discontinuity, or Moho. Option A is correct because seismologists use changes in the speed and behaviour of P and S waves to locate such internal boundaries. Option B, length of a river, gives surface geographical information and cannot reveal a deep underground boundary. Option C, forest area, describes vegetation, not Earth’s internal structure. Option D, crop yield, depends on soil, weather, and farming, so it is irrelevant to the mantle. Thus the evidence must be connected with waves travelling through the Earth. Memory cue: internal layers are detected by seismic-wave changes, not surface activities.

Volcanic activity brings lava and other materials to the surface. Exam tip: consider it a direct natural source.

Direct access to deep parts is difficult so indirect clues are needed. Exam tip: write this reason clearly.

Direct sources are useful but do not reach great depth. Exam tip: write both limitation and use.

Seismic waves help infer layers without digging. Exam tip: consider it the most useful indirect method.

Scientists form conclusions by comparing many evidences. Exam tip: remember the importance of combined evidence.